Given g(x)=∣f(x)∣ and f(c)=0g′(c+)=x→c+limx−c∣f(x)∣−f(c)=x→c+lim∣x−cf(x)−f(c)∣,asx>c
⇒g′(c+)=∣f′(c)∣
g(c−)=x→c−limx−c∣f(x)∣−f(c)=−x→c−lim∣x−cf(x)−f(c)∣,asx<c
⇒g′(c−)=−∣f′(c)∣
For g(x) to be differentiable at x=c, f′(c) must be 0. Else, it is non-differentiable.